In the practice of liquid chromatography and other analytical methods involving flowing liquid streams, it is a common requirement to analyze samples which are available only in quantities less than one milliliter in volume. Furthermore, the columns employed for the analyses often require injection of small sample volumes, typically one to fifty microliters, in order to operate efficiently. It has become common practice to use sample injection devices which accept variable volumes of sample, transferred into the injector via an injector opening which accepts the needle of a micro syringe. The syringe serves to measure the volume of the introduced sample. The sample then resides in a chamber, often referred to as the sample loop, which is at atmospheric pressure. Upon subsequent actuation of the injector device, the sample is swept at a high pressure, of typically 100 to 10,000 psi, from the chamber and into the column by the flow of mobile phase from the pump.
One type of prior art injector device has a needle-receiving passage from which the syringe is withdrawn subsequent to the transfer of sample into the chamber, and which must be plugged prior to the actuation of the injector. The plug prevents loss of high pressure fluid from the port. Such devices are described in U.S. Pat. Nos. 3,916,692 by Abrahams et al and 3,961,534 by Gundelfinger. One advantage of such devices is that all of the sample transferred by the syringe is subsequently injected into the column, so there is no sample loss. This is beneficial because the syringe calibration marks on a conventional syringe provide an accurate indication of the volume actually injected into the column. Other advantages are that there is no waste of the sample, which is sometimes valuable and in limited supply, and there is no left over sample in the injector which could cause cross-contamination with the next sample to be injected. However, such injector devices have the disadvantage of requiring the needle passage to be plugged after syringe removal and prior to actuation. Removing the syringe needle and inserting the plug can lead to volumetric errors, since the sample chamber experiences the exiting and entering of the needle and plug probes. Furthermore, the entire process is somewhat complicated and tedious.
A second type of prior art injector device, that can be utilized without withdrawing the syringe or plugging the passage, is described in U.S. Pat. Nos. 4,022,005 by Ramin et al and 4,068,528 by Gundelfinger. However, such devices have the disadvantage that some of the sample transferred from the syringe, remains in the flow passage which connects the syringe needle to the sample chamber. As a consequence, some sample is wasted, and the left over sample can be a source of cross-contamination between samples. Such cross-contamination can be avoided by extensively flushing the connecting passage with a pure solvent between every injection, but this is inconvenient as it requires a second syringe to be inserted into the needle port. Another disadvantage is that the calibrations on a standard syringe do not accurately indicate the volume of sample injected into the column, since no account is taken of the trapped sample portion.. The user can factor in this error, or use a special syringe having calibrations offset to account for sample loss, but neither of these is as convenient as using a standard syringe and simple calculations.
An object of this invention is to provide an injector device which is easy to use, avoids wastage of sample, avoids cross-contamination of samples, and accurately delivers a sample to a chromatographic column in accordance with standard syringe calibrations.